In traditional processes used today in industry, wood is treated by chemical aqueous solutions such as alkaline or alkaline earth salts of sulphydric acid or sulphurous acid.
These techniques have the following disadvantages;
duration of cooking of several hours, up to ten hours,
Considerable dilution of the aqueous solutions which represent 5 to 10 times the weight of wood used, whence the necessity to have a considerable volume of water available and to use a great amount of energy for heating to the cooking temperature.
The black liquid subproduct must be evaporated so as to reach a concentration of the order of 50% of dry matter, allowing combustion and chemical products to be recovered, whence a considerable energy consumption considering the mass of water used.
High investment costs due to the volumes and the service pressure of the reactors.
Pollution of the atmosphere by gaseous sulphurated subproducts (dimethylsulfide, methylmercaptan).
Pollution of the waters by liquid cooking waste.
Other processes which have not reached the industrial level consist in treating the wood by solvents of the alcohol, amine, aminoalcohol, sulfoxide type in an acid medium or in the presence of alkaline or earth alkaline bases.
These processes while have certain advantages with respect to the traditional processes such as a higher cooking rate (30 minutes to 3 hours) and absence of pollution, are however difficult to justify economically, because of the excessively high consumption of the solvent used and the price thereof.
This consumption is itself relates partly to the volume of solvent used (2 to 6 times the weight of the wood).
An improvement for reducing the consumption of solvent consists in treating the wood shavings in a twin-screw mixing extruder with a minimum of solvent and caustic soda for example, so as to obtain a pasty consistence.
In this improved process, although the loss of solvent is reduced it is however not neglible and the equipment to be used must be specially tuned so as to avoid in particular a premature cellulose fiber-black liquid separation in the body of the extruder, which would lead to blockages.
The process of the invention allows most of the above-mentioned disadvantages to be avoided by cooking the wood shavings at atmospheric pressure or at a very low pressure in the solid phase after impregnation with a chemical reactant.